The invention relates to a device with a magnetic position sensor comprising a field sensor and analysis electronics for linear displacements of a rod-shaped component, in particular the shaft of an actuator, with an element generating a magnetic field and a position sensor measuring the magnetic field strength angle of this field, the field angle signal determined by this sensor being used for displacement path determination.
U.S. Pat. No. 5,570,015 discloses a magnetic position measurement device with which linear displacements of a rod-shaped component, for example a valve shaft, can be measured. A particular measurement unit, which consists of a shaft used for measurement purposes only and the position sensor, is placed on the rod-shaped component whose displacements are to be measured. Let into the shaft of the measurement unit is a rod-shaped magnet which generates a magnetic field in the longitudinal direction of the shaft. The position sensor is arranged fixedly in the area of the magnetic field. On relative displacements of the shaft in relation to its position, the position sensor measures by means of field sensors which, using analysis electronics, determine the magnetic field strength angle approximately proportionally to the displacement path. The path proportionality exists however only within a certain range.
Apart from the large space required, the drawback of this arrangement is that the shaft must not turn. If shielding is used, the device no longer works sufficiently linearly and is therefore no longer sufficiently precise.
It is an object of the invention to provide a device with a magnetic position sensor which is simpler and smaller in structure, but in particular performs its measurements contactlessly and directly on the rod-shaped component of the actuator.
According to the invention, this object is achieved in that the element generating a magnetic field is an axially magnetized magnet casing which surrounds the rod-shaped component of the actuator itself, is fixedly connected thereto and can rotate therewith about its displacement axis.
If the element generating the magnetic field is attached directly to the shaft of the actuator, there is no need for a separate shaft of the measurement unit. Another advantage is that the shaft can rotate about its own axis without this affecting the measurement. This is very important because, in this case, a round actuator shaft does not require a twist-resistant guide for measurement. For faultless operation, it is advantageous if the actuator shaft consists of a non-ferromagnetic or only a weakly ferromagnetic material. The position sensor is also applicable in the case of inaccessible shaft ends.
With regard to the magnet casing length there is a long measurement range. This allows a compact construction with a non-temperature-sensitive measurement principle. There is also a high linearity of correlation between position and measurement signal. An additional analysis device, using a sensor curve to increase the precision, can be dispensed with in this case.
In a further embodiment of the invention, the field sensor comprises magnetoresistive elements, Hall effect sensors or field coils.
For the measurement accuracy it may be desirable to change the magnetic fieldxe2x80x94although not during operation but in the context of certain configurationsxe2x80x94in relation to that of a single magnet casing. In a further embodiment of the invention, the magnet casing consists of a magnetic material of preferably axially joined ring discs.
The assembly of discs allows further variants. In a further embodiment of the invention, the ring discs consist of materials having a different remanence.
If an assembly of discs is not desired, the magnet casing in a further embodiment of the invention is magnetized axially differently when consisting of a homogeneous magnetic material. In these forms of magnet casings, the measurement accuracy is considerably improved.
In a further embodiment of the invention, the magnet casing, either consisting of a homogeneous material or assembled from ring discs has a ratio of diameter to length ranging from 2/3 to 3/2, preferably in the proximity of 1, so as to form a minimum field strength.
In a further embodiment of the invention, the field sensors are arranged on the inner side of a casing-shaped screen. This screen may have a round or a square cross-section.
In a further embodiment of the invention, the whole position sensor i.e. both the field sensors and the analysis electronics, are situated within the screen. This combination is extremely simple and flexible.
In a further embodiment of the invention, the magnet casing is let into a ring-shaped recess of the rod-shaped component and the inlet casing is surrounded by a non-ferromagnetic shaft casing of greater length. The shaft casing can also be let into the rod-shaped component.
In a further embodiment of the invention, the rod-shaped component is surrounded by a non-ferromagnetic linear guide casing, in the casing opening of which it can slide with the magnet casing, and the non-ferromagnetic linear guide casing is surrounded by a casing-shaped screen within which the field sensor is situated.
In a further embodiment of the invention, the analysis electronics are arranged on the outer side of the screen and carry the sensors within the linear guide casing by means of a carrier guided through the screen.
In a further embodiment of the invention, the rod-shaped component in the area of the measurement device consists of two parts connected together by means of a central pin arrangement, and the magnet casing and the shaft casing are pushed onto the rod-shaped component in the area of the pin arrangement.
Thus, a very simple and suitable and compact measurement device is obtained which can be used favorably, in particular in the automotive sector.